There exists today a group of steels which are characterized by among other things enhanced mechanical properties including higher yield strengths and tensile strengths than plain carbon structural steels. These are known as high-strength, low-alloy (HSLA) steels. Different types of HSLA steels are available, some of which are carbon-manganese steels and others of which are microalloyed by additions of such elements as niobium, vanadium, and titanium to achieve enhanced mechanical properties. The original demand for HSLA steels arose from the need to obtain improved strength-to-weight ratios to reduce dead weight in transportation equipment. In addition to the original uses, HSLA steels are used today in a wide range of applications including vehicles, construction machinery, materials-handling equipment, bridges and buildings.
Commercial HSLA steels typically have minimum yield strengths of 40 to 50 ksi and minimum tensile strengths of 60 to 70 ksi. The mechanical properties and other characteristics of HSLA steels are set forth in standard specifications such as SAE J410c. Microalloyed HSLA steels have even higher strengths on the order of minimum yield strengths of 50 to 80 ksi and minimum tensile strengths of 65 to 95 ksi. These steels use additions of alloying elements such as niobium, vanadium, titanium, zirconium and rare earth elements in concentrations generally below 0.10 to 0.15% to achieve higher strength levels. Heat treatment is not involved because the properties of microalloyed HSLA steels result from controlled rolling on continuous hot strip mills.
One grade of microalloyed, high-strength, low-alloy steel under SAE J410c is grade 970X, which is characterized by a minimum yield strength (0.2% offset) of 70,000 psi, minimum tensile strength of 85,000 psi, and minimum elongation (2-inch specimen) of 14%. As stated, this material exhibits its mechanical properties as hot rolled. When later cold reduced to sheet thickness, these steels are subjected to a low temperature recovery anneal for an extended period of time to maintain the controlled rolled mechanical properties. In addition to the increased cost because of the addition of microalloying elements, this recovery anneal is disadvantageous because of either the extended times required for box annealing or the enormous investment required for equipment for continuous annealing.
There thus exists today a need for steels possessing th desired combination of strength and ductility required for HSLA steel applications but which can be produced economically from cold reduced sheet stock without the need for extended recovery annealing. Moreover, there exists a need for such steels wherein the higher mechanical properties, particularly yield strength and tensile strength, are achieved without the intentional inclusion of microalloying agents such as niobium, titanium and vanadium, which otherwise would add significantly to the cost of the steel.